1. Field of the Invention
The present invention relates generally to an electrophotographic printing apparatus such as laser printers, photo copy machines, and regular paper facsimile machines. More particularly, it relates to a transfer roller of a printing apparatus which includes first rollers and second roller having different resistance properties in order to prevent a large amount of toner from adhering the ends of its charging roller and photoconductive drum.
2. Description of the Related Art
Varying the resistivity along the length of the transfer roller is not new in the art. U.S. Pat. No. 4,379,630 for a Transfer Roller For Electrophotographic Apparatus to Suzuki '630 discloses a transfer roller with a resistance value that varies in either a continuous or a stepwise manner toward the respective end faces at two end portions of the transfer roller. Suzuki uses as a varying resistance transfer roller to eliminate the problems called multiple copy fog phenomenon and multiple-copy edge line phenomenon. The multiple copy fog phenomenon and the multiple copy edge line phenomenon can be suppressed by using a transfer roller whose resistivity at the ends are different from the resistivity at the center. Unlike the arrangement in the present invention, Suzuki '630 uses a transfer roller and a photoconductive drum whose lengths are less than the width of a sheet of recording medium conveyed therebetween. As a result, Suzuki addresses and tries to correct different problems than that of the present invention. The present invention relies on the fact that the transfer roller and the photoconductive drum are longer than the width of recording medium.